1. Field of the Invention
The present invention relates to a fuel cell system with hydrogen storing alloy from which hydrogen gas is introduced to an anode of a fuel cell unit.
2. Description of the Prior Art
A fuel cell system comprises in general a fuel cell unit, an air supplying means and a fuel supplying means. The fuel cell unit has an electrolyte membrane such as a proton exchange membrane (PEM) between two electrodes, that is a cathode to which an atmospheric air (or oxygen) is supplied through the air supplying means and an anode to which hydrogen gas is supplied through the fuel supplying means. Oxygen at the cathode and hydrogen at the anode react with each other to generate electricity. In actual application, the system includes a plurality of fuel cells which are stacked in series with a separator being interposed between adjacent fuel cells.
An attempt has been proposed that a hydrogen storing alloy is used as a hydrogen gas supply source. As known, the hydrogen storing alloy is capable of storing therein and discharging therefrom hydrogen gas depending upon the ambient temperature and the partial pressure of hydrogen therearound. The hydrogen storing alloy stores hydrogen gas in an exothermic reaction whereas it discharges hydrogen gas in an endothermic reaction. For example, in a fuel cell system disclosed in Japanese un-examined patent publication No. 7-192743, water circulates between the fuel cell and the hydrogen storing alloy. More particularly, water is heated after it is used to cool down the fuel cell and then fed to around the hydrogen storing alloy which is activated by the heated water to discharge hydrogen gas which is supplied to the anode of the fuel cell. Water of a lowered temperature as a result of the endothermic reaction of the hydrogen storing alloy is returned to the fuel cell for use as a coolant.
Although this prior art system provides improved heat circulation throughout the fuel cell system, water circulation comprises various components including conduits, pumps, valves and radiators. It also requires energy which is a part of electricity generated by the fuel cell unit in the system.
It is therefore an object of the present invention to provide a fuel cell system capable of eliminating disadvantages of the above-described prior art technology.
Another object of the present invention is to provide a fuel cell system which is simple in construction, small in size, easy to install and, therefore, particularly suitable to be mounted on a vehicle.
According to an aspect of the present invention there is provided a fuel cell system comprising one or more fuel cell units each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; a hydrogen storing alloy from which hydrogen gas is discharged, when heated, to be supplied to the anode of the fuel cell unit; and heating means for heating the hydrogen storing alloy by contact with an exhaust gas from the fuel cell unit.
With this fuel cell system, the hydrogen storing alloy is heated by the exhaust gas which has been heated during operation of the fuel cell unit, thereby producing hydrogen gas. In other words, heat in the exhaust gas enhances the endothermic reaction of the hydrogen storing alloy in which hydrogen gas stored therein is discharged which is supplied to the anode of the fuel cell unit.
The fuel cell unit in the fuel cell system of the present invention may be of any suitable arrangement and construction. By way of example, it comprises a proton exchange membrane (PEM) between the cathode and the anode. PEM acts as an electrolyte and transports therethrough hydrogen ions obtained at the anode of the fuel cell toward the cathode, in the form of proton (H+). The hydrogen storing alloy used in the fuel cell system of the present invention includes LaNi5, TiFe, ZrMn2, Mg2Ni and any other alloy which is capable of discharging hydrogen gas stored therein, when heated. For example, LaNi5 is known to provide endothermic reaction LaNi5H6xe2x86x92LaNi5+3H2, when heated to about 50-80xc2x0 C., causing hydrogen gas to be produced by about 300 liters per hour.
A casing of the hydrogen storing alloy should preferably have a greater a surface area which facilitates heat transmission from the discharge gas. In a preferred embodiment, the casing has a plurality of apertures which allows the discharge gas to enter the interior of the casing. In another preferred embodiment, there are a plurality of tubular casings each containing the hydrogen storing alloy.
The exhaust gas to be used in this invention for heating the hydrogen storing alloy may be either one of the remaining air discharged from the cathode and the remaining hydrogen gas discharged from the anode.
The air discharged from the cathode contains water (vapor) generated by the fuel cell reaction. Accordingly, where the hydrogen storing alloy is heated by the discharged air from the cathode of the fuel cell unit, it is preferable that a condenser is arranged downstream of the hydrogen storing alloy for cooling down the discharged air to collect water. The water collected by the condenser may be reused to moisten the air to be supplied to the cathode. Since, in this invention, the discharged air has been cooled to at least some extent by heat exchange with the hydrogen storing alloy for achieving the endothermic reaction, the condenser may be subjected to a decreased load.
In a preferred embodiment of the fuel cell system of this invention, liquid water is supplied to the cathode. More specifically, liquid water is sprayed directly onto the cathode, which is hereinbelow referred to by xe2x80x9cwater spray type fuel cell systemxe2x80x9d. In the water spray type fuel cell system, the exhaust gas from the cathode has a greater water content than that from the anode. A part of water in the exhaust gas is collected by heat exchange with the hydrogen storing alloy and the remainder is collected by the condenser.
In accordance with another aspect of the present invention there is provided a fuel cell system comprising one or more of fuel cell units each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; a hydrogen storing alloy which produces hydrogen gas, when heated, to be supplied to the anode of the fuel cell unit; and water spray means for spraying liquid water onto the cathode. The sprayed water is then supplied to the hydrogen storing alloy so as to cool the hydrogen storing alloy to thereby enhance exothermic reaction thereof in which hydrogen gas is stored in the hydrogen storing alloy. The sprayed water functions to suitably moisten the electrolyte membrane such as PEM during operation of the fuel cell unit. In this aspect of the present invention, the sprayed water also functions to cool the hydrogen storing alloy while the fuel cell unit is at a standstill, which facilitates the exothermic reaction by which the hydrogen storing alloy is filled with hydrogen gas. This system makes it unnecessary to provide separate cooling means for cooling the hydrogen storing alloy, or at least minimizes the energy for driving such a separate cooling means.
In accordance with still another aspect of the present invention there is provided a fuel cell system comprising one or more fuel cell units each having an anode, a cathode and an electrolyte membrane interposed between the anode and the cathode; an exhaust port arranged below the fuel cell unit for passing therethrough exhaust gas from the cathode of the fuel cell unit; a suction port arranged below the exhaust port for passing therethrough an air to be supplied to the cathode of the fuel cell unit; and a partition member between the exhaust port and the suction port for effecting heat exchange between the exhaust gas in the exhaust port and the air in the suction port. With this system, the exhaust gas from the cathode having an elevated temperature may be cooled by heat exchange with the air in the suction port having a lower temperature. The exhaust gas from the cathode of the fuel cell unit is relatively weighty because it contains water, which may smoothly fall down with gravity toward the exhaust port. This is particularly suitable when applied to the water spray type fuel cell system in which liquid water is sprayed directly onto the cathode, which will fall down together with the exhaust gas from the cathode. The term xe2x80x9cexhaust portxe2x80x9d includes an exhaust manifold and any chamber or passage through which the exhaust gas from the cathode is discharged to the open air, whereas the term xe2x80x9csuction portxe2x80x9d includes any chamber or passage through which the open air is introduced to the cathode of the fuel cell unit.